1. Field of the Invention
The present invention relates to a measuring device for measuring the size of an object by advancing or retracting a spindle by screwing with respect to the object, that is, a measuring device represented, for example, by micrometers and micrometer heads.
2. Description of Related Art
A conventionally known measuring device represented, for example, by micrometers and micrometer heads consists of a body with a female thread and a spindle with a male thread, the female and male threads engaging with each other, and measures the size of an object by advancing or retracting the spindle with respect to the object via screwing dependent on the engagement of the two threads.
With such a measuring device, the axial displacement for each revolution of the spindle (single turn axial displacement) is determined by the pitch of male threads inscribed on the spindle.
The male threads on the spindle of a conventional measuring device generally have a pitch of 0.5 or 0.635 mm.
Some measuring devices have been known that have a spindle whose thread has au enlarged pitch to increase its single turn axial displacement.
With a measuring device whose spindle has a male thread with a pitch of 0.5 or 0.635 mm, however, measurement becomes very cumbersome if the measuring device must measure many objects different in size successively, because then the spindle must travel a considerable distance each time one object is exchanged for another, and the spindle must be revolved many times to cover the distance because of its single turn axial displacement being small.
To meet such inconvenience, if the thread of a spindle is made to have a larger pitch, the axial interval between the ridge and trough of each thread must be increased, and for this purpose a sufficiently large amount of material constituting the spindle must be bitten away during preparation of the spindle. This will in turn lead to the reduced strength of the spindle. Such a spindle is prone to bend during preparation, and thus may cause the precise machination of threads to be impaired.
Moreover, if the thread of a spindle is made to have a large pitch while other things being kept invariable, the number of male threads on the spindle to engage with the female threads on a body will be reduced, which may reduce the resistance of the threads against a force applied on the axis of the spindle, and thus may cause the precision of axial displacement of the spindle to be impaired. This problem may be circumvented by increasing the length of the male and female threads such that the number of male threads to engage with the female threads remains invariable. However, such countermeasure will cause the overall size of the measuring device to be increased.